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Abstract:

The present invention relates to a six bed pressure swing adsorption
(PSA) system utilizing new and advanced cycles to obtain enhanced
hydrogen recovery from a hydrogen containing feed gas (i.e., synthesis
gas). In one such cycle each of the six beds has four pressure
equalization steps, and at least one of the beds is receiving and
processing said feed gas to obtain a hydrogen product gas (i.e., a 6-1-4
cycle).

Claims:

1. A pressure swing adsorption process for separating a pressurized
supply feed gas containing one or more strongly adsorbable component from
at least one less strongly adsorbable product gas component in a six bed
adsorbent system to produce a continuous stream of product gas enriched
in the less strongly adsorbable component and a stream of offgas that is
enriched in the strongly adsorbable components, wherein the duration of
lowest pressure equalization down and first blowdown step is less than
15% of the feed time, and the process cycle has at least eighteen steps
including four bed-to-bed equalization steps.

3. The pressure swing adsorption process of claim 1, wherein the six bed
system is in turndown mode with five beds online, where the process cycle
has fifteen steps including three bed-to-bed equalizations steps, while
one of the beds is in production.

7. The pressure swing adsorption process of claim 6, where the carbon and
zeolite layers are each subdivided into two layers with different
particle size.

8. The pressure swing adsorption process of claim 7, where the first of
the subdivided carbon layers encountered by the supply feed gas has a
particle size of about 0.5 to 1.5 mm and an affinity for the carbon
dioxide impurity.

9. The pressure swing adsorption process of claim 7, where the second of
the subdivided carbon layers encountered by the supply feed gas has a
particle size of about 2.0 to 3.0 mm and an affinity for the methane
impurities.

10. The pressure swing adsorption process of claim 7, where the first of
the subdivided zeolite layers encountered by the supply feed gas has a
particle size of about 0.5 to 2.0 mm and an affinity for the carbon
monoxide impurity.

11. The pressure swing adsorption process of claim 7, where the second of
the subdivided zeolite layers encountered by the supply feed gas has a
particle size of about 2.0 to 3.0 mm and an affinity for the nitrogen
impurity.

12. A pressure swing adsorption process for separating a pressurized
supply feed gas containing one or more strongly adsorbable component from
at least one less strongly adsorbable product gas component in a five bed
adsorbent system to produce a continuous stream of product gas enriched
in the less strongly adsorbable component and a stream of offgas that is
enriched in the strongly adsorbable components, wherein the duration of
lowest pressure equalization down and first blow down step is less than
15% of the feed time, and the process cycle has at least fifteen steps
including three bed-to-bed equalization steps.

Description:

[0001] This application is a continuation-in-part of prior U.S.
application Ser. No. 13/004,706, filed Jan. 11, 2011, which is
incorporated herein by reference.

FIELD OF THE INVENTION

[0002] The present invention relates to a six bed pressure swing
adsorption (PSA) system utilizing new and advanced cycles to obtain
enhanced hydrogen recovery from a hydrogen containing feed gas (i.e.,
synthesis gas). In one such cycle each of the six beds has four pressure
equalization steps, and at least one of the beds is receiving and
processing said feed gas to obtain a hydrogen product gas (i.e., a 6-1-4
cycle). The six bed PSA system can be operated in a turndown mode where
one or two beds are taken offline. The new five bed cycle includes three
equalization steps, while at least one bed is in operation (i.e., a 5-1-3
cycle). The four bed cycle includes two equalization steps, while at
least one bed is in operation. This turndown mode, achieves a reduction
of less than four percent, and eight percent reduction, respectively, in
hydrogen throughput. In addition, the invention relates to a novel
adsorbent layering employed in the beds of the PSA.

BACKGROUND OF THE INVENTION

[0003] The need for high purity gasses, such as hydrogen, processed from
effluents in the chemical process industries remain. These effluents are
hydrogen containing feed mixtures gases (i.e., synthesis gases), from the
likes of steam methane reforming of natural gas or naptha, catalytic
reforming of hydrocarbons, isomerization processes, etc, which are routed
to a PSA for further processing. This growing demand requires the need to
develop highly efficient separation processes (e.g., PSA) for hydrogen
(H2) production from various feed mixtures. In order to obtain
highly efficient PSA separation processes, both the capital and operating
costs of the PSA system must be reduced. Some ways of reducing PSA system
cost include a decrease in the adsorbent inventory, reduction in the
number of PSA beds, and using advanced cycles in the PSA processes. The
aforementioned ways of reducing the PSA system cost constitute the
elements of the present invention.

[0004] Conventional PSA systems are well known for separating feed mixture
gases which contain components with different adsorption characteristics.
For example, in a typical PSA system, a multicomponent gas mixture is
passed to at least one of multiple adsorption beds at an elevated
pressure to adsorb at least one strongly sorbed component while at least
one component passes through. In the case of hydrogen PSA, hydrogen is
the most weakly adsorbed component which passes through the bed. At a
defined time, the feed step is discontinued and the adsorption bed is
co-currently depressurized in one or more steps, counter-currently purged
in one or more steps and counter-currently blown down in one or more
steps to permit essentially pure hydrogen product to exit the bed with a
high recovery. The sequence of steps is not limited to the one stated
above and a combination of two or more steps can be employed as a single
step as well.

[0005] U.S. Pat. No. 4,834,780 is directed to six bed PSA system having
one bed in operation of the 6-1-3 and 6-1-4 cycles, wherein the first
number in the cycle refers to the total number of beds, the second number
refers to the number of beds on the feed step at any instant, and the
third number refers to the number of bed to bed equalization steps in the
PSA cycle. Thus, 6-1-3 means a six bed PSA cycle having one bed on feed
at any instant, and the PSA cycle contains three bed-to-bed equalization
steps. This cycle is reproduced in Table 1, below.

[0006] In accordance with the teachings of the 6-1-3 cycle, the PSA system
delivers a continuous flow of PSA tail gas to a surge drum that removes
fluctuations in the pressure, flowrates and compositions, prior to
sending it to an upstream unit (e.g., SMR) for use (e.g., SMR burner
fuel). However, the 6-1-4 cycle of the patent features a discontinuous
PSA tail gas flow routed upstream via the surge drum. The discontinuous
PSA tail gas flow can create fluctuations in the hydrogen plants.

[0007] U.S. Pat. No. 6,454,838, is directed to a modified 6-1-4 cycle to
remove the undesired discontinuous PSA tail gas supply to the surge drum.
However, the solution provided in this patent results in a PSA cycle
having several idle steps. Specifically, a twenty four step cycle is
described with four idle steps and overlapping fourth equalization and
blowdown steps. Those skilled in the art would recognize that idle steps
in the PSA cycle invariably result in degradation in PSA process
performance (e.g., lower hydrogen recovery). In an alternative
embodiment, U.S. Pat. No. 6,454,838 discloses a 6-1-4 PSA cycle wherein
the PSA cycle consists of twenty four steps in the cycle (See Table 3)
featuring the following: (1) overlapping provide purge and the fourth
equalization step; (2) additional tank for the temporal storage of gas
from the second equalization step; (3) no idle steps and (4) continuous
off-gas flow. However, in this embodiment a storage tank is utilized in
order to eliminate the four idle steps. Furthermore, the PSA process
recovery drops by 1-1.5% for cycles utilizing a fourth equalization
compared to the prior art 6-1-3 cycle. See Table 4.

[0008] U.S. Pat. No. 6,007,606, co-owned by the assignor of the present
invention, discloses a PSA process involving the storage of products
having various purities in segregated storage tanks for subsequent usage.
Products of increasing purities, admitted at the product end of the bed
are used during purging and re-pressurization steps. In addition,
different composition streams collected at the feed end of the bed during
the countercurrent depressurization step are admitted at the feed end of
the bed, in the order of increasing product component content during the
rising pressure step(s).

[0009] In addition to the cycles, the related art discusses conventional
adsorbent materials utilized in the beds as a means for improving the
product recovery in hydrogen PSA systems. For example, U.S. Pat. No.
6,814,787 is directed to PSA apparatus and process for the production of
purified hydrogen from a feed gas stream containing heavy hydrocarbons
(i.e., hydrocarbons having at least six carbon atoms). The apparatus
includes at least one bed containing at least three layers. The layered
adsorption zone contains a feed end with a low surface area adsorbent (20
to 400 m2/g) which comprises 2 to 20% of the total bed length
followed by a layer of an intermediate surface area adsorbent (425 to 800
m2/g) which comprises 25 to 40% of the total bed length and a final
layer of high surface area adsorbent (825 to 2000 m2/g) which
comprises 40 to 78% of the total bed length.

[0010] U.S. Pat. No. 6,340,382, is directed to a PSA process that purifies
hydrogen from a mixture that passes through an aluminum oxide
(Al2O3) layer for moisture removal, then through activated
carbon layer for carbon dioxide (CO2), carbon monoxide (CO), and
methane (CH4) removal, and finally through CaX zeolite layer for
nitrogen (N2) removal to produce high purity H2 (>99.99%).
CaX is at least 90% Ca exchanged with SiO2/Al2O3=2.0.

[0011] U.S. Pat. No. 7,537,742 B2 relates to an optimum set of adsorbents
for use in hydrogen PSA systems. Each adsorbent bed is divided into four
regions. The first region contains adsorbent for removing water. The
second region contains a mixture of strong and weak adsorbents to remove
bulk impurities like CO2. The third region contains a high bulk
density (>38 lbm/ft3) adsorbent to remove remaining CO2 and
most of CH4 and CO present in the hydrogen containing feed mixtures.
The fourth region contains adsorbent having high Henry's law constants
for the final cleanup of N2 and residual impurities to produce
hydrogen at the desired high purity.

[0012] U.S. Pat. No. 6,402,813 B2 describes the purification of a gas
mixture by adsorption of the impurities on carbon adsorbent formed by a
combination of several different active carbons. In particular, a PSA
process is described for purifying a gas, such as hydrogen, nitrogen,
oxygen, carbon monoxide, argon, methane or gas mixtures containing these
components. The gas stream to be purified is passed through layers of
carbons, wherein the ordering of the carbon layers are such that at least
one of the following conditions exist: (1) the density (D) is such that
D1<D2; (2) the specific surface area (SSA) is such that SSA1>SSA2;
3) the mean pore size (MPS) is such that MPS1>MPS2, and (4) the pore
volume is such that PV1>PV2. More specifically, this patent relates to
a process in which at least two layers of activated carbons are used in
which the first layer carbon has lesser density than the second, the
first carbon has more specific surface area, and also more mean pore size
than the second carbon.

[0013] To overcome the disadvantages of the related art six bed PSA
systems, it is an object of the, present invention to introduce new and
advanced PSA cycles with turndown modes, which include all of the
following features (1) no need for additional storage tank; (2) no idle
steps (3) higher recovery than the related art cycles.

[0014] It is another object of the invention to modify the adsorbent
system in each bed to contain at least three layers of adsorbents (e.g.,
alumina, activated carbon and zeolite), wherein the active carbon and
zeolite components are layered based on particle size and enables
additional improvement in hydrogen recovery. Therefore, an efficient PSA
separation process has been found with high hydrogen recovery, lower
adsorbent requirements, (i.e., lower bed size factor (BSF)), and lower
capital and operating costs. Additionally, the process should operate
efficiently when one or more beds are taken offline for operational
reasons such as valve failure (referred herein, as "turndown" or
"turndown mode").

BACKGROUND OF THE INVENTION

[0015] The invention provides a pressure swing adsorption process for the
separation of a pressurized feed gas supply containing one or more
strongly adsorbable components and at least one less strongly adsorbable
product gas in a multiple bed system. The feed gas is supplied to a feed
end of an adsorbent bed containing solid adsorbent material(s), which
preferentially adsorbs the more strongly adsorbable component(s) and
withdrawing the least strongly adsorbable product component from an exit
end of the adsorber bed, producing in cycle including steps in which the
continuous feed gas sequentially co-currently flows through each of the
adsorber beds to produce gas product using continuous feed gas,
pressurization steps, pressure equalization steps, blowdown step(s), and
purge step(s).

[0016] The product gas of the process is preferably hydrogen although the
process can be extended to other separation processes such as helium
purification, natural gas upgrading, CO2 production from synthesis
gas or other source containing CO2 in the supply feed or in other
PSA processes for coproduction of H2 and CO2. One of the novel
features of the present invention is the introduction of a new and
advanced cycle to a six bed PSA system having four equalization steps to
achieve enhanced H2 recovery. This cycle can be further modified and
utilized to operate the PSA system in a turndown mode with a relatively
small reduction in throughput, thereby allowing the PSA system to operate
with as few as four beds, yet maintaining a throughput of hydrogen above
90%. Another novel feature of the invention is the layered adsorbent,
which can be utilized in the beds. These layered configurations of carbon
and/or zeolite components differ from another layer of similar adsorbent
material in particle size. These layered, configurations of the bed
materials combined and the PSA cycles provide a synergistic effect with
an overall improvement in hydrogen recovery and throughput of 1-2% over
conventional PSA cycles.

BRIEF DESCRIPTION OF THE FIGURES

[0017] The objects and advantages of the invention will be better
understood from the following detailed description of the preferred
embodiments thereof in connection with the accompanying figures wherein:

[0018]FIG. 1 illustrates an advanced bed configuration/layering in
accordance with one aspect of the invention;

[0019] FIGS. 2A is a plot of the relative adsorption rates of N2 and
CO versus particle diameter for the zeolite layers shown in FIG. 1;

[0020]FIG. 2B is a plot of the relative adsorption rates of CO2 and
CH4 versus particle diameter for the carbon layers shown in FIG. 1;
and

[0021]FIG. 3 is an illustrative six bed H2 PSA system/skid utilized
with the cycles of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0022] The invention discloses high efficiency PSA processes/cycles
employed in a six bed PSA system to attain 20-50 million standard cubic
feet per day (MMSCFSD) of hydrogen production. The cycles achieve
enhanced recovery of hydrogen from a hydrogen containing gas. The
invention provides the following features:

[0023] Novel and advanced PSA cycles for six bed PSA systems which can be
operated in turndown mode, and provide high hydrogen recovery.

[0024] The novel six bed PSA cycle has at least four bed-to-bed
equalization steps; and

[0025] The elimination of idle steps in the cycles and no need for
segregated storage tanks.

[0026] Another aspect of the invention concerns the adsorbents loaded into
the beds of the hydrogen PSA to enhance the recovery of hydrogen. It has
been found that the three layers of adsorbents where in each layer is
subdivided into two layers containing the same adsorbent, yet with
different particle size, optimal adsorption and desorption kinetics for
specific impurities present in the hydrogen containing feed gas is
attained. This advanced adsorbent layering configuration thereby results
in an improvement in hydrogen recovery.

[0027] Typical hydrogen PSA process utilizes three different adsorbents
loaded in the vessel from the bottom to the top in the order such as (1)
alumina; (2) activated carbon and (3) zeolite. There are five major
impurities to be removed by adsorption process. Alumina adsorbs the
moisture contained in the feed gas. Activated carbon layer is usually
designed to take care of carbon dioxide and hydrocarbons such as methane,
ethane, and propane. The zeolite function is to remove carbon monoxide,
nitrogen, argon and residual methane not taken out by activated carbon
placed upstream of the zeolite. Additional details of the layers of
adsorbents in each PSA bed are discussed in Baksh et al (U.S. Pat. No.
7,537,742 B2), which is co-owned by the assignee of the present
invention, and is incorporated by reference herein, in its entirety.

[0028]FIG. 1 is illustrative of the adsorbents layers in each of the PSA
beds of the invention. The adsorption properties in layers two, three,
four and five are fine tuned by optimizing the particle size of the
adsorbent used to achieve optimal PSA process performance. As an example,
layers two and three are identical (e.g., both are the same carbon
material) except for the difference in particle sizes. Likewise, layers
four and five are identical (i.e., both are the same zeolite material),
but their particle size is different. The adsorber vessel design and
configuration is such that it will be capable to adsorb five different
components. Ideally, layer 1 adsorbs moisture, layer 2 adsorbs carbon
dioxide, layer 3 adsorbs methane, layer 4 adsorbs carbon monoxide and
layer 5 adsorbs nitrogen. Those skilled in the art will recognize that
the process recovery will be maximized when adsorbents are fully
utilized. Using a three layer design, the skilled artisan has only three
degrees of freedom to size the adsorber for removal of five components.
The inventive approach adds two more degrees of freedom thus making it
possible to achieve higher hydrogen recovery in combination with the new
6-1-4 cycle of this invention.

[0029] The adjustment of the adsorbent particle size affects the rate of
adsorption and desorption process--the adsorption capacity is independent
of particle size. The diffusion resistance in an adsorption process is
the sum of all diffusion resistances within the particle of the adsorbent
material. The change in the particle size may or may not affect the
overall diffusion resistance depending on the level of contribution of
the diffusion phenomena affected by the particle size.

[0030] In one embodiment, CaX(2.3) zeolite is used in the fourth and fifth
layers of FIG. 1. The layers are sized such that layer four preferably
adsorbs carbon monoxide and layer five preferably adsorbs nitrogen. With
reference to FIG. 2A, the dependence of relative adsorption rates on
particle diameter for both nitrogen and carbon monoxide is shown. The
Zero Length Column (ZLC) technique is employed to obtain the data plotted
in FIG. 2A. See, J. A. C. Silva & A. E. Rodrigues, Gas. Sep. Purif., Vol.
10, No. 4, pp. 207-224, 1996.

[0031] The value of the relative adsorption rate is the ratio of true
adsorption rate and the standard rate value. The standard rate value
corresponds to a required minimum rate needed to yield the enhanced PSA
process performance. If the same particle size (e.g., 2.5 mm) is used for
both layers in the process the requirement for nitrogen rate is
satisfied. However, as can be determined from FIG. 2A, the CO relative
adsorption rate is only 40% of the required minimum. Therefore, it is
desirable to decrease the zeolite particle size in layer four in order to
increase the rate of carbon monoxide adsorption. A value of 1.5 mm meets
the design specification for the rate of carbon monoxide in this
particular exemplary embodiment. It is obvious that one could increase
the nitrogen rate as well by decreasing the particle size in layer five.
As a result, only negligible process improvement will be realized since
the nitrogen adsorption rate is already above the required minimum value.
On the other hand; the process performance can suffer from the increased
pressure drop in the bed. The preferred layering for this particular
example will be particle sizes larger than 2 mm and smaller than 3 mm for
layer five and particle sizes larger than 0.5 mm and smaller than 2 mm
for layer four.

[0032] Carbon layers two and three will be occupied with the carbon
particles of different size as well. The ZLC technique is employed once
again to measure the adsorption rates for carbon dioxide and methane on
the carbon material. The rate data normalized by the standard rate is
summarized in FIG. 2B. The rate for methane is satisfactory at particle
sizes less than 2.25 mm. However, the smaller particles are needed to
obtain reasonable rates for carbon dioxide. By inspection of the data in
FIG. 2B, the preferred carbon particle size for carbon dioxide take out
is less than 1.5 mm and for methane less than 2 mm. Thus the layering for
this particular example will be particle sizes larger than 1.0 mm and
smaller than 2.0 mm for layer three and particle sizes larger than 0.5 mm
and smaller than 1.5 mm for layer two.

[0033] The novel PSA cycles of the present invention will now be described
with reference to various exemplary embodiments. In one embodiment of the
invention, the novel PSA system employs an eighteen-step, six adsorbent
bed PSA cycle having four equalization steps, in addition to purging,
blowdown and product pressurization steps (referred herein as "the 6-1-4
PSA cycle"). The PSA system includes a continuous supply of feed gas to
at least one bed which in adsorption. This bed separates the pressurized
supply feed gas containing one or more strongly adsorbable component and
allowing the less strongly adsorbable hydrogen product gas to exit the
bed.

[0034] In another embodiment of the invention, the PSA system can be
utilized in turndown mode with five beds. The PSA cycle for the five beds
would include fifteen steps, where one bed is in adsorption and has three
equalization steps in addition to the purging and product pressurization
steps (referred herein as "the 5-1-3 PSA cycle").

[0035] In an alternative embodiment of the invention, the PSA system
having six beds employs an eighteen steps cycle where two of the beds are
simultaneously in the adsorption phase, and each bed has at least three
equalization steps in addition to purging and product pressurization
steps referred herein as "the 6-2-3 PSA cycle").

[0036] With reference to FIG. 3 and Tables 2 and 3, the mode of operation
for the 6-1-4 PSA cycle is illustrated. Specifically, the sequence of
steps for the 6-1-4 PSA cycle is performed in the order recited in each
of the adsorption vessels in turn.

[0037] It will be understood that the nomenclature provided for this 6-1-4
PSA cycle is the same for all the cycles discussed herein, where:

[0038] A1=First Adsorption Step

[0039] A2/PP1=Second Adsorption Step/First Product Pressurization

[0040] A3/PP2=Third Adsorption Step/Second Product Pressurization

[0041] E1=First Equalization Down

[0042] E2=Second Equalization Down

[0043] E3=Third Equalization Down

[0044] PPG=Provide Purge Gas

[0045] E4/BD1=Fourth Equalization Down/First Blowdown

[0046] BD2=Second Blowdown

[0047] PG=Purge

[0048] E4'=Equalization Up (using gas from E4 step)

[0049] E3'=Equalization Up (using gas from E3 step)

[0050] E2'=Equalization Up (using gas from E2 step)

[0051] E1'=Equalization Up (using gas from E1 step)

[0052] PP1=First Product Pressurization

[0053] PP2=Second Product Pressurization

[0054] In some of the cycles, as the cycle may require, the following
additional nomenclature shall be employed:

[0055] A2=Second Adsorption Step

[0056] A3=Third Adsorption Step

[0057] A4=Fourth Adsorption Step

[0058] A5=Fifth Adsorption Step

[0059] PP=Product Pressurization

[0060] A3/PP1=Third Adsorption Step/First Product Pressurization Step

[0061] E3/BD1=Third Equalization Down/First Blowdown Step

[0062] In Table 2, the rows correspond to a particular bed in the PSA
process while the columns represent the step number. The duration of one
cycle sequence (one row) is referred to as the total cycle time or cycle
time (CT). The cycle time is constant for each bed. The relative shift in
the cycle steps among the beds can be inferred from Table 2 as well. This
shift is equal to 1/6th of the CT since there are six beds in this
particular cycle. In order for the 6-1-4 PSA cycle to be fully defined
the step times for step 1, 2 and 3 must be assigned--such as t1,
t2 and t3. The duration of the basic block, also referred to as
the feed time, is then defined as t1+t2+t3. Employing the
cycle periodicity described above, the CT=6*(t1+t2+t3) and
it follows that the duration of steps 1, 4, 7, 10, 13 & 16 are equal in
time (i.e., t1); steps 2, 5, 8, 11, 14 & 17 (i.e., t2) and
steps 3, 6, 9, 12, 15 & 18 (i.e., t3). Hence, there are eighteen
steps in the cycle, the mode of operation for each bed is offset by three
steps.

[0063] The 6-1-4 PSA cycle sequence is now described with respect to one
bed which undergoes the entire PSA cycle. A representative PSA train/skid
system having six beds in parallel is depicted in FIG. 3, and is employed
herein to illustrate this embodiment. The system includes 30 on/off
valves, and 14 control valves, 6 manifolds (although the sixth manifold
is labeled "7" in FIG. 3 in order to designate the PSA skid components
with consistent nomenclature, as set forth below) and associate pipings
and fitting. The control valves are utilized to control the flow rate or
pressure during certain process steps while the on/off valves allow
communication between the various beds in the PSA system. The valve
sequencing representing the steps in the 6-1-4 PSA cycle of FIG. 3 is
illustrated in Table 3, below, where the valve chart defines the position
or action for each valve (i.e., open=O, closed=C, and CV=control valve)
in a particular step of the PSA cycle.

[0064] Step No. 1 (A1): The feed gas mixture is introduced to the bottom
of Bed 1 from the feed manifold at high pressure. Both valves 011 (i.e.,
XV-011) and 012 (i.e., XV-012) are open while all other Bed 1 valves
(XV-01x) are closed. Hereinafter valve tag numbers will be referred to
without using the prefix XV. The feed mixture flows from the bottom to
the top of the bed. This upward flow direction in the vessel is referred
to as co-current flow with respect to feed. During this adsorption step
the impurities are adsorbed and high purity hydrogen is collected and
routed through product manifold number 2. Control valve 002 is used to
control the pressure in the bed in (A1), (A2) or (A3) steps (i.e. the
feed/production steps).

[0065] Steps No. 2 and 3 (A2/PP1 and A3/PP2): These steps are identical
except for their respective duration t2 and t3. Valves 011 and
012 remain open, and Bed 1 continues in the feed/production step. In
addition, control valve 007 is used to control the rate of product
pressurization steps (PP1) and (PP2). As shown in Table 2, above, it is
the Bed 2 that is receiving the product pressurization gas from Bed 1 in
Steps No. 2 and 3 through valves 007, 027 and 028. All other valves
associated with Bed 2 are closed. It is important that the pressures in
Bed 1 and Bed 2 are equal at the end of (A3/PP2) step so that the Bed 2
can enter the feed/production step (A1) in Step 4. It is also desirable
that the (PP1) and (PP2) step flow rates are regulated by valve 007 to be
as low as possible in order to prevent the fluidization and to keep the
pressure in Bed 1 as high as possible.

[0066] Step No. 4 (El): Bed 1 undergoes the first bed-to-bed equalization
step while Bed 3 is counter currently receiving the equalization
gas--step (E1'). The (E1) step is sometimes referred to as co-current
depressurization step. Bed 1 valves 017, 018 and Bed 3 valves 037 and 038
are open while all other Bed 1 and Bed 3 valves (i.e., 01x and 03x) are
closed. The rate of (E1)-(E1') steps is controlled by control valve 018.

[0067] Steps No. 5 and 6 (E2): Bed 1 undergoes the second equalization
(E2) for the duration of these two steps. Specifically, the pressure in
Bed 1 drops due to co-current gas flow from Bed 1 to Bed 4 undergoing
step (E2') during these steps. The pressures in both beds are equal at
the end of step No. 6. Valves 015, 045 and 048 are fully open while valve
018 controls the rate of (E2)-(E2') steps.

[0068] Step No. 7 (E3): Bed 1 executes the third equalization step (E3).
This step uses the same equalization manifold as the previous step (E2).
Valves 015, 055 and 058 are fully open while valve 018 controls the rate
of (E3)-(E3') steps. It is clear from the valve nomenclature that Bed 5
is in communication with Bed 1 using manifold number 5.

[0069] Step No. 8 (PPG): Bed 1 co-currently provides purge gas to Bed 6,
which is being purged. To this end valve 013 is open and control valve
018 is used to control the rate of the (PPG) step. The purge gas flows to
the purge manifold number 3 to Bed 6 while valves 063 and 068 are fully
open. Counter-current purge step uses hydrogen rich stream to aid the
regeneration of adsorbents in the vessels. Desorbed impurities leave the
Bed 6 through the control valve 064 and eventually are collected in a
surge drum (not shown). From the operational standpoint, the longer the
purge step the better the adsorbent regeneration and thus better process
working capacity. This means that the purge step time (t2) should be
long and as mentioned above the product pressurization step time
(t2+t3) should be long as well, if permitted by process. This
condition is easily satisfied for 6-1-4 cycle shown in Table 2 since the
(PG) and (PP1) steps overlap; their step time is t2.

[0070] Step No. 9 (E4/BD1): This step is the fourth equalization step (E4)
coupled with a blowdown step (BD1) executed sequentially. At the
beginning of step 9, valves 013, 063 and 068 are open and valve 018 is
used to control the rate of the fourth equalization step. Proper
management of adsorption/desorption processes taking place in this step
is essential for the superior 6-1-4 cycle performance. The gas used for
the co-current depressurization step (E4) must be rich in hydrogen. This
is achieved by advanced layering technology (i.e. the use of CaX type
zeolite adsorbent while larger particles located at the top of the vessel
and smaller particles below), discussed in detail above. Once Bed 1 and
Bed 6 have reached pressure equalization (i.e., EQ4 is completed), the
above mentioned valves associated with EQ4 step are closed and valve 014
is opened to enable the blowdown step where a portion of the gas in Bed 1
is directed to the surge drum (not shown). The (BD1) flow rate is
controlled by valve 014. As a consequence, this Step 9 of 6-1-4 cycle
does not generate any offgas for the duration of EQ4 step. The offgas is
generated only during the BD1 portion of step 9. The duration of this
lowest pressure equalization and blow down step (i.e., EQ4/BD1) is less
than 15% of the feed time (i.e., (t1+t2+t3)).

[0071] Step No. 10 (BD2): This step in Bed 1 is carried out to rid the
vessel of the impurities adsorbed during co-current steps (AD, EQ, PPG)
through the bottom of the vessel. At this point in the cycle, the
pressure in the vessel is too low to hold on to the impurities. As a
result, they are desorbed and counter-currently directed to the surge
drum through valve 014. All other valves associated with Bed 1 are closed
during this step.

[0073] Step No. 12 (E4'): The first step designated as equalization up to
reference the bed receiving the gas. The beds in (E4/BD1) and (E4') steps
are interacting such that the content of Bed 2 is transferred to Bed 1
until the pressures in both beds is equalized. Valves 023, 013 and 018
are fully open and the action of control valve 028 provides means to
control the rate of this step.

[0074] Step No. 13 (E3'): Bed 1 is receiving gas from Bed 3. Valves 015,
035 and 018 are fully open and the action of control valve 038 provides
means to control the rate.

[0075] Steps No. 14 and 15 (E2'). Bed 1 is receiving gas from Bed 4.
Valves 015, 045 and 018 are fully open and the action of control valve
048 provides means to control the rate.

[0076] Step No. 16 (E1'): The last equalization step where Bed 1 receives
the gas from Bed 5. Valves 017, 057 and 018 are fully open and the action
of control valve 058 provides means to control the rate.

[0077] Steps No. 17 and 18 (PP1 and PP2): The last two steps in the cycle
description with regards to Bed 1 are the (PP1) and (PP2) steps already
described above.

[0078] Basic functionality of the cycle can be described in the same
fashion for any bed. However, once the step sequence for one bed is
defined the step sequences for other beds will follow in the same order
and the relative time shift will be 1/6th of the cycle time (CT),
(e.g., Bed 2 starts the adsorption (A1) in step number 4, compared to Bed
No. 1 that goes through (A1) in step number 1).

[0079] During the operation of a plant employing a six bed PSA process it
may be desirable to operate the plant in the turndown mode for a limited
period of time. In the case of a six bed/vessel PSA system, this mode
enables the continuous production with only five vessels online while one
of the beds or valves associated with a given bed failed and need to be
serviced. It is often seen in the industry that the plant performance
significantly deteriorates when operating in exceptional mode. With
reference to Table 4, below, the mode of operation of the new 5-1-3 PSA
cycle is described.

[0080] In order for the 5-1-3 PSA cycle to be fully defined the step times
for step 1, 2 and 3 must be assigned--such as t1, t2 and
t3. The duration of basic block is then defined as
t1+t2+t3. By using the cycle periodicity the total
CT=5*(t1+t2+t3) and it follows that the duration of steps
1, 4, 7, 10 & 13 equal to t1; steps 2, 5, 8, 11 & 14 equal to
t2 and steps 3, 6, 9, 12 & 15 equal to t3. The cycle sequence
will be described bellow in detail with respect to Bed 1 for illustration
purposes assuming that Bed 6 is offline and completely isolated from the
rest of the process. Functionality of the cycle is explained using
hydrogen the PSA process valve skid shown FIG. 3. The 5-1-3 PSA cycle
sequence is now described with respect to one bed which undergoes the
entire PSA cycle (i.e., CT).

[0081] Steps 1 and 2 (A1 and A2): Bed 1 begins the process cycle in the
adsorption steps (A1) and (A2). Both valves 011 and 012 are open while
all other Bed 1 valves (01x) are closed; the high purity hydrogen is
collected and sent through the product manifold number 2. Control valve
002 is used to control the pressure in the bed in (A1), (A2) or (A3)
steps (i.e. all feed/production steps).

[0082] Step 3 (A3/PP1): Valves 011 and 012 remain open, Bed 1 continues in
the feed/production step. In addition, the control valve 007 is used to
control the rate of product pressurization step (PP1). Therefore, Bed 2
is receiving the product pressurization gas from Bed 1 through valves
007, 027 and 028. All other valves associated with Bed 2 are closed.

[0083] Steps 4 and 5 (E1): Bed 1 undergoes the first bed-to-bed
equalization step (E1) while Bed 3 is counter-currently receiving the
equalization gas--step (E1'). Bed 1 valves 017, 018 and Bed 3 valves 037
and 038 are open while all other Bed 1 and Bed 3 valves (01x) and (03x)
are closed. The rate of (E1)-(E1') steps is controlled by control valve
018.

[0084] Step 6 (E2): The pressure in Bed 1 drops due to gas flow from Bed 1
to Bed 4 undergoing step (E2'). The pressures in both beds are equal at
the end of step 6. Valves 015, 045 and 048 are fully open while valve 018
controls the rate of (E2)-(E2') steps.

[0085] Step 7 (PPG): Bed 1 sends purge gas to Bed 5 in the purge step
(PG). Valve 013 is open and control valve 018 is used to control the rate
of (PPG) step. The purge gas flows through the purge manifold number 3 to
the Bed 5 while valves 053 and 058 are fully open. Desorbed impurities
leave the Bed 5 through the control valve 054 and eventually are
collected in the surge drum (not shown).

[0086] Step 8 (E3/BD1): In Bed 1, the (E3) step coupled with a blowdown
step (BD1). Both ends of Bed 1 are open. Valve 014 is opened to enable
the blowdown step where a portion of the gas in Bed 1 is directed to the
surge drum. In the meantime, valves 015, 055 and 058 are open and valve
018 is used to control the rate of third equalization step. The duration
of this lowest pressure equalization and blow down step (E3/BD1) is less
than 15% of the feed time.

[0087] Step 9 (BD2): The purpose of this step is to rid the vessel of the
impurities adsorbed during co-current steps (AD, EQ, PPG) through the
bottom of the vessel via valve 014. All other valves associated with Bed
1 are closed during this step.

[0088] Step 10 (PG): Is a purge step where Bed 1 is receiving the purge
gas from Bed 2. Valves 018, 013, 023 are fully open. The rate of (PPG)
step and the pressure in Bed 1 is controlled via valves 028 and 014
respectively.

[0089] Step 11 (E3'): This first equalization up step designates that the
bed is receiving the gas. The beds in (E3) and (E3') steps are
interacting such that the content of Bed 2 is transferred to Bed 1 until
the pressures in both beds are equalized. Valves 025, 015 and 018 are
fully open and the action of control valve 028 provides means to control
the rate.

[0090] Step 12 (E2'): Bed 1 is receiving gas from Bed 3. Valves 015, 035
and 018 are fully open and the action of control valve 038 provides means
to control the rate.

[0091] Step 13 and 14 (E1'): Bed 1 receives gas from Bed 4. Valves 017,
047 and 018 are fully open and the action of control valve 048 provides
means to control the equalization step rate in steps 13 and 14.

[0092] Step 15 (PP1): The last step in the cycle description with regards
to Bed 1 where the product pressurization occurs, as described above.

[0093] The five bed PSA system, it may be desirable to further operate the
plant in a turndown mode with only four beds/vessels online. In such a
case, the 4-1-2 PSA cycle of Baksh et al. (U.S. Pat. No. 6,340,382) is
utilized and incorporated by reference in its entirely.

[0094] An example, a PSA process with the cycles described herein were
simulated under the process conditions listed in Table 5, below. The
model assumes the following feed gas composition for all cycles: 73.87%
hydrogen, 0.23% nitrogen, 3.31% carbon monoxide, 16.37% carbon dioxide,
5.94% methane and 0.3% water. The feed gas temperature was 100° F.
and feed gas pressure was 360 Psig.

[0095] As shown in the Table 5, the new cycles having an additional
equalization step, without the need for idle steps or otherwise offline
storage tanks provides for a recovery of hydrogen as high as 88.0%. On
the other hand, in the turndown mode with five and four beds online, the
hydrogen recovery drops to 85.5% and 81.0%, respectively.

[0096] An alternative embodiment of the present invention is a six bed PSA
system having dual feed. One of the benefits of using a dual feed cycle
is higher throughput and lower bed size factor (BSF). As discussed with
respect to the cycles above, BSF is a measure of process productivity per
ton of hydrogen produced per day. This alternative PSA cycle has eighteen
steps, two beds are simultaneously on the feed/process stage and three
steps in the cycle are dedicated to bed-to-bed equalization. This 6-2-3
PSA cycle has an increased throughput capability of producing 40-70
MMSCFD of hydrogen versus 20-50 MMSCFD for 6-1-4 PSA cycle. The remainder
of the innovative characteristics of the 6-2-3 PSA cycle are the same as
the ones discussed with respect to the newly designed 6-1-4 and 5-1-3 PSA
cycles.

[0097] The 6-2-3 PSA cycle has eighteen steps with two parallel feeds and
three bed to bed equalization steps. Since six beds are used with an
eighteen step cycle (18/6), three cycle steps and their times (t1,
t2, t3) must be described for full cycle definition. An
alternative way for describing a cycle chart is to provide information on
all of the beds for the duration of the unit block rather than describing
the whole sequence for a single bed. For example, by defining all cycle
steps in steps 1, 2 and 3 for 6-2-3 PSA cycle in Table 6, one has
qualitatively defined all possible interactions among beds, valves and
manifolds. The same sequence will be periodically repeating with period
equal to t1+t2+t3. This new method will be used to explain
the functionality of 6-2-3 PSA cycle with reference to Table 6, below, in
conjunction with FIG. 3.

[0098] Step No. 1: Two beds are processing feed (adsorption step), namely
Bed 1 and Bed 6. It follows that the valves 011, 012, 061 and 062 will be
open. Bed 5 and Bed 2 are in communication executing (E1)-(E1') steps,
where valves 057, 027, 028 are open and valve 058 is used to control the
rate. Bed 4 is providing the purge gas (PG) for Bed 3. The rate of the
(PPG) step is controlled by valve 048, while valves 043, 033, 038 are
fully open and valve 034 is used to control the pressure in Bed 3.

[0099] Step No. 2: Two beds are processing feed (adsorption step), namely
Bed 1 and Bed 6. Thus, it follows that valves 011, 012, 061 and 062 are
open. Bed 5 and Bed 2 are continuing in equalization steps (E1)-(E1'),
valves 057, 027, 028 are open and valve 058 is used to control the rate.
Bed 4 and Bed 3 are undergoing (E3/BD1)-(E3') steps (i.e., overlapping
low pressure equalization step and blowdown--(BD1). The duration of this
lowest pressure equalization and blow down step (i.e., E3/BD1) is less
than 15% of the feed time. Valves 045, 035, 038 are open and valve 048 is
used to control the flow rate of gas to Bed 3. At the same time, Bed 4 is
blown down through valve 044 and (BD1) step gas is directed towards the
surge drum via manifold number 4.

[0100] Step No. 3: Two beds are processing feed (adsorption step), namely
Bed 1 and Bed 6. Thus, it follows valves 011, 012, 061 and 062 will be
open. Bed 2 is in the product pressurization step (PP). Valves 027 and
028 are open while valve 007 controls the rate of this step. Bed 5 and
Bed 3 are in communication executing (E2)-(E2') steps, valves 055, 035,
038 are open and valve 058 is used to control the rate. Bed 4 is in the
blowdown step, when the bed is counter-currently depressurized and its
content is released to the surge drum through valve 044 and manifold
number 4.

[0101] The performance for the 6-2-3 PSA cycle was obtained via
mathematical modeling of hydrogen PSA processes. The results are
summarized in Table 7. The model assumed following feed gas composition
for all cycles: 73.87% hydrogen, 0.23% nitrogen, 3.31% carbon monoxide,
16.37% carbon dioxide, 5.94% methane and 0.3% water. The feed gas
temperature was 100° F. and feed gas pressure was 360 Psig. The
configuration of vessels and adsorbents used were exactly same as those
considered for single feed cycles (i.e., See Table 5).

[0102] The simulation results show that the benefit from new 6-2-3 PSA
cycle is one additional point in hydrogen recovery. Thus, in both cycles
reported in Table 7, the adsorbent layering of the present invention has
been incorporated into the cycles. Accordingly, the performance of 6-2-2
cycle in Table 7 does not correspond to prior art performance. The
recovery of prior art H2 PSA process using 6-2-2 cycle without the
advanced layering of the adsorbents is the range of 82 to 83%.

[0103] The major advantage of 6-2-3 cycle versus 6-1-4 cycle is the lower
bed size factor (BSF). The BSF is 5618 lbs/TPDH2 for the 6-1-4 PSA
cycle (Table 5) and 4583 lbs/TPDH2 for the 6-2-3 PSA cycle (Table 7),
respectively. As a result, the 6-2-3 PSA cycle can produce a greater
quantity of hydrogen using smaller amount of adsorbents.

[0104] While the invention has been described in detail with reference to
specific embodiment thereof, it will become apparent to one skilled in
the art that various changes and modifications can be made, and
equivalents employed, without departing from the scope of the appended
claims.